This chapter gives an overview of epidemiology and the role epidemiological studies have played in many different aspects of the AIDS epidemic. The chapter first focuses on epidemiological principles and methods and then provides current surveillance data (as of 2004) tracking the progress of the AIDS epidemic in the USA and around the world.
In the introductory section, epidemiology is defined as the study of the patterns of disease occurrence in populations and of the factors affecting them. It is pointed out that epidemiological studies give information about populations, not about specific individuals. A comprehensive list of what epidemiology can tell us about diseases, including identifying new diseases, populations at risk, and possible causative agents is provided.
The next section describes how important epidemiological studies were in the early days of the AIDS epidemic in identifying AIDS (or GRID as it was then known) as a new disease and identifying homosexual men as the initial risk group. Epidemiological studies were also crucial in identifying a pattern of occurrences suggesting that AIDS was caused by a sexually-transmitted infectious agent and later that AIDS could be transmitted by contaminated blood.
Basic concepts in epidemiology are clearly described. The terms prevalence and incidence are defined and the use of these terms are explained. The two basic kinds of epidemiological studies are outlined: descriptive and analytical; the first describing the occurrence of disease in populations by categories of person, place and time; the second seeking to identify and explain the causes of disease. This is followed by specific examples of each type of study. Descriptive studies include case reports/case report series while analytical studies include experimental/interventional studies and observational studies, either case/control or cohort studies. In analytical studies the results are considered in terms of a statistical association or correlation between a factor and a disease. In this way causality between a factor and a disease, for example that smoking causes lung cancer, can be established. The possible interpretations of a correlation between a factor and a disease are carefully explained and the criteria for a causal relationship are described.
The last part of chapter 6 contains extensive AIDS-related epidemiological data supported by graphs and diagrams describing the progression of the AIDS epidemic in the USA and the rest of the world. The data shown includes cumulative AIDS cases in the USA through 2004, distribution of AIDS cases in the USA by risk group, and AIDS cases in the USA by ethnicity, as well as a discussion of risk factors and non-risk factors for HIV infection. The changing face of the HIV/AIDS epidemic in the USA, Africa, and Asia is clearly documented, as are new areas of rapid spread. Finally, a short discussion on the different clades and subgroups of HIV is given.
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